Correct method of stacking sandbags to prevent flooding. Image: U.S. Army Corps of Engineers-Walla Walla.

Forecasts call for a wet week in the Northwest. The National Weather Service expects several more inches of rain across Western Washington in the next few days. Flood watches and warning are a strong possibility.

One of the best all-around websites on all things weather in Western Washington is the Take Winter by Storm website — a public-private sector sponsored information source.

The heavy rains forecasted this weekend may cause more than just localized flooding and higher river levels. Prolonged or intense rainfall increases the chances of shallow landslides on hillsides and other steep slopes. During these rain events, some of the rainwater flows across the surface to nearby streams and rivers, some is captured by plants and other vegetation, and some of the water infiltrates the ground. With enough rainwater infiltrating the ground, soils can weaken and slide.

Think of building sand castles with buckets on the beach–with the right amount of water, the grains of sand bind together to form a near-perfect cast of the bucket, but if too much water is added, the sand cannot hold its form and collapses under its own weight. Soil saturation has a similar result on a steep slope. With enough rain, the soil becomes saturated and begins to lose strength, increasing the chances of a landslide.

The geology of western Washington — steep slopes and soils — make this landslide country but with the right conditions, steep slopes in eastern Washington are vulnerable, too. Lots of rain, combined with failing drainage systems and development that increases surfacewater runoff near steep slopes, can be landslide triggers on both sides of the Cascades.

Image of a shallow landslide that initiated during a prolonged and intense rain event in Thurston County. (Image Courtesy of Stephen Slaughter, DNR)

Warning signs of an impending landslide

If you live on or near a steep slope, here are some warning signs of potential slope instability:

Cracks forming in your yard, driveway, sidewalk, foundation or in other structures.

Trees on slopes, especially evergreens, start tilting.

Doors and windows suddenly become more difficult to open or close.

Water begins seeping from hillsides, even during dry weather.

If you see any of these early signs of a potential landslide, immediately contact your city or county.

The combination of weak, rain-soaked soils on sloped terrain led to this mud slide in January 2009 that scattered mud, rocks and other debris across State Route 542 east of Deming. Photo: WSDOT

Washington State’s climate , topography, and geology create a perfect setting for landslides, especially along our hillsides and shorelines. In Western Washington, most landslides happen in fall and winter, which is when we tend to get hit by big storms that bring heavy rainfall totals like those we’ve seen in the past couple of days. Our landslide hazard map, still in testing phase, shows the current risks based on recent rainfall totals.

DNR collects landslide data for several reasons. As the population of Washington grows, there are increasing pressures to develop housing and businesses in landslide-prone areas. Knowing the location of these landslide hazards has never been more important to public safety.

DNR’s Geology and Earth Resources Division is helping to increase understanding of landslide processes in Washington State through numerous research projects. As the state’s official geological survey, DNR seeks to provide information about landslide risks that can help counties and citizen when making decisions about land use.

The National Weather Service is predicting heavy rainfall across much of western Washington, including the Cascade Mountains, today through Friday afternoon. Amounts could range from one-inch in the lowlands to three inches or more in the foothills and mountains. Such intense rainfall in a short period of time could cause mud or rock slides as well as flooding.

Here in Washington, a combination of climate, topography, and geology creates a perfect setting for landslides. In western Washington, most landslides happen when we get heavy rainfall. Are you prepared? The Washington Emergency Management Division has useful information to help you prepare for natural hazards of all types, including storms, flooding and landslides.

Here are some the common warning signs of an impending mud or rock slide (the fast-moving, ‘shallow’ landslide that can be caused by heavy rains):

Sudden decrease in creek water levels, especially during storms

Sudden increase in creek water levels, often with increased sediment in the water

Sounds of cracking wood, knocking boulders, or groaning of the surrounding ground, or unusual sounds, like the sound of an oncoming freight train, especially if the sound increases

A hillside that has increasing springs, seeps, or saturated ground, especially if it has been dry

Formation of cracks or tilting of trees, especially evergreens, on a hillside

One of the most effective things you can do to keep safe in landslide country during landslide ‘weather’ is to know where landslides have occurred in the past. DNR’s Geology and Earth Resources Division collects landslide data to help increase understanding of landslide processes in Washington state. Take a look at our growing database on the Washington State Geologic Information Portal.

DNR geologist Stephen Slaughter photographs the newly exposed hillside at the site of the Whidbey Island landslide on March 27. Photo: Isabelle Sarikhan/DNR. More photos on Flickr.

LAST UPDATE: 6:25 p.m., 3/28/2013

Incident: Early in the morning on 3/27/2013, along Driftwood Way in the Ledgewood Beach Community in Coupeville, Whidbey Island, residents awoke to a noise that sounded like thunder or a sonic boom. A large landslide caused damage that was hundreds of feet wide. Initial reports are that seven homes were threatened and 17 more are isolated. No one was injured.

A Department of Natural Resources (DNR) Division of Geology and Earth Resources team is working with local emergency responders and the Island County Emergency Management Division to survey this landslide and understand the incident and its risks.

Key points from the report include:
– The Ledgewood-Bonair Landslide (LB landslide) on Whidbey Island, Island County, Washington occurred around 3:45 am on March 27, 2013.
– It is a small portion of a much larger landslide complex, approximately 1.5 miles long, that was prehistoric and may date back as far as 11,000 years.
– The top of the landslide scarp averages 200 feet above sea level.
– The landslide pushed (uplifted) the beach as high as 30 feet above the shore.
– The toe (front of landslide at the beach) is slightly over 1,100 feet long and extends approximately 300 feet into Puget Sound.
– Uplift of the beach is presumed to have been relatively slow (i.e., over a few minutes).
– Wave and tidal action is actively eroding the toe with small sections (1-10 cubic feet) observed calving with the rising tide.
– The volume of material moved was approximately 200,000 cubic yards (equivalent to 40,000 dump truck loads).
– DNR geologic mapping from 2009 shows the extent of the landslide complex (Polenz and others, 2009, http://www.dnr.wa.gov/Publications/ger_gm68_geol_map_camano_24k.pdf).
– Where observed, the access road has been shifted approximately 80 feet down vertically and to the west.

The volume of material moved was approximately 200,000 cubic yards (equivalent to 40,000 dump truck loads).Photo by: DNR/Stephen Slaughter March 27, 2013

The latest series of slides should not come as a complete surprise: slopes along this stretch of track are steep, unstable, and have a history of failure. Plus, this is the time of year when saturating rains move across Western Washington. Slides like the ones along the tracks north of Seattle (one was caught on amateur video) are typically caused by intense or prolonged rainfall that soaks thin soils lying atop an impenetrable substrate and weak rock joints — prime conditions for slope failure.

DNR’s mapping of landslides shows that the coastal rail route north of Seattle has a history of mudslides (see the green areas marked on the map). We document landslides to help citizens, industry and government agencies to understand landslide risks at a location so they can take steps to protect people and property from harm.

Several counties in Western Washington had moderate or high risks of mudslides as of Wednesday afternoon. Image: DNR/NOAA

Many people living near Belfair in Mason County woke up to cold, darkened houses this morning after a landslide pushed trees and debris into power lines and equipment about 10 miles north of the town. The slide may have been due to locally heavy rains. Yes, it’s that time of year again: rain in Western Washington and, sooner or later, storms on the Eastside.

A test project by DNR and the National Weather Service is trying a raise awareness of mudslide risks, particularly the ones that occur because of heavy rain or prolonged periods of rain. Our online beta (test) map combines recent rainfall measurements from Weather Service rain gauges with information about the local soils and mudslide history. The result is a county-wide risk level rating. After more testing and refinements, this map may eventually be able to show mudslide risks in smaller areas than county-wide.

Take a look and see how your county rates now. The map is not intended to predict mudslides at any particular time or location; it only rates the overall risk that one might occur based on the amount of rain that has just fallen. The map is still in its testing phase, but we have placed it on the DNR website so you can be better informed.

The Washington State Geologic Portal is an online tool to locate geologic features and resources.

National Earth Science Week (October 14-19) concludes today with a special focus on geologic mapping and its importance to society. Geologic Map Day aims to put the spotlight on the uses of geologic maps for education, science, business, and a variety of public policy concerns. For example, DNR’s Geology and Earth Resources Division maps areas in Washington susceptible to soil liquefaction and other hazards. These maps help planners, builders and citizens to better understand the potential risks that earthquakes may post to buildings, roads, utilities and other human-constructed features.

Another map — the online Shallow Landslide Hazard Map shows the latest risk levels for mudslide by county. Still in beta testing but available to view, this online map is a joint project between DNR, the National Weather Service/NOAA.

Or you can really dig down (sorry, pun intended) with DNR’s Washington State Geologic Information Portal. These interactive maps allow you to create, save, and print custom geologic maps for almost any location in the state. You can even search by street address if you want. You also can download map data for use in a geographic information system (GIS) from the portal, which features several ‘pre-loaded’ maps focusing on landslides, tsunami evacuation, geothermal resources, subsurface geology, Mount Rainier lahar hazards and more. Here’s a 2-page fact sheet explaining how to get the most out of the Washington State Geologic Information Portal.